This invention relates to data communication systems and more particularly to data communication systems in a computer based education (CBE) environment.
CBE has been in development for the past 25 years and in use, on a limited basis, for the past 20 years. The acknowledged state of the art in CBE is the PLATO system which was first developed at the University of Illinois and is described in U.S. Pat. No. 3,405,457, the contents of which are incorporated herein by reference.
A number of articles have considered various aspects of the PLATO System and its viability in CBE, e.g. see: "Advances in Computer-based Education" by Alpert and Bitzer, Science, 167 1582-1590 (March 1970)
PLATO is a large, main-frame central processor system which can service over a thousand connected display terminals on a real-time, interactive basis. The central processor makes available to the student/user tens of thousands of hours of courses in a variety of subject areas, in the form of pedagogical material displayed on the students' terminals.
The student typically has available an electronic display (such as a CRT) and a keyboard. Information is presented in the form of text and/or graphical matter (akin to a textbook). The student, via a keyboard, can respond by indicating he/she has completed review of the material being presented and is ready for presentation of additional material. The student can further manifest an understanding of the material by responding to questions and the like.
Speed of response by the central computer to student questions and instructions is of great importance: students will not sit idly by while the machine spends many seconds in presenting a new display, or responding to keyboard operations. In short, successful CBE interaction requires fractional second response and correspondingly fast screen displays.
Terminals employed in the PLATO systems may be relatively "dumb", in that most application programs are executed in the central computer while display programs are executed in the terminal. Most data transmission between the central computer and associated terminals consists of transmitting to the terminal, via telephone lines, information necessary to produce a display, and transmitting from the terminal to the computer on the same lines, the user's keyboard entries.
Several major cost factors have historically mitigated against making central computer CBE services available, on an economic basis, to the potential mass of users. The first has been the intrinsic hardware costs of the central processors and allied equipment. In the recent years, however, equipment providing comparable and in many cases, better performance have been produced at highly reduced costs through technological innovation. However, the other major cost element, communications, have not decreased and in some cases have increased.
Telephone line tariffs characteristically double or triple the basic cost of a terminal's monthly connection charge. For instance while it is not unusual to pay between $250-$500 per terminal per month for access to a central computer CBE system, telephone line charges can raise that monthly cost to $1000-$1200 per terminal per month. These high communication charges are present even though a number of terminals may share a single telephone line.
The limiting factor to the number of terminals that can share a telephone line is the data rate which the line can accommodate (e.g. 9600 bits per second). Characteristically in a CBE system, it is forward channel data flow (ie. computer to terminal) which controls rather than reverse channel data flow (i.e. terminal to computer). For instance, forward channel data flow in a PLATO CBE system averages 250-300 bits per second per terminal. With a loading factor of approximately 0.7 on a 9600 bps line, 16 to 20 terminals can be multiplexed on a single telephone line. In contrast, reverse channel data flow in such a system is much slower, averaging less than 20 bits per second per/terminal. This low rate of data transmission is a result of the reverse channel data flow being almost exclusively keystroke data to which is appended proper routing information.
Another factor which has limited the number of terminals connected to a telephone line is the rapid response time required to satisfy the user. A successful system should appear to the student to give "instantaneous" replies to the student's queries and instructions. This response characteristic requires that there be little or no queueing, polling or other communications protocols which require the user to sit and wait for a "turn". An average system response time to the student of a few tenths of a second (e.g. approximately 0.3 sec) needs to be attained before a system can truly be called "interactive".
In order to reduce communications costs inherent in present central processor based CBE systems, modes of data transmission other than telephone lines have been explored. In U.S. Pat. No. 4,633,462, a protocol for the use of existing CATV communication channels is described which preserves the rapid response time to the user.
Forward channel data transmission using locally generated television style signals have also been employed by the PLATO system at the University of Illinois with reverse channel communications being via telephone lines. That system employed a time slotted, data transmission protocol which greatly reduced the amount of traffic that could be handled. In specific, each terminal in the system was allocated a time slot and the TV signal was multiplexed on a time division basis. Thus, if there was no data to be transmitted to a terminal or less data than the time division could accommodate, the empty portion of the time slot was wasted. That system was very limited in the number of terminals it could accommodate. Furthermore, the TV signal generation was essentially line of sight, limited the potential user base, and was costly since it had to be reproduced at each central site along with all of the other central computing apparatus.
Another communication mode, i.e. via satellite has been explored. Until recently satellite transponder space has been scarce and costly. This is no longer the case. A T1 channel (1.544 megabits/sec) is now available at reasonable cost. However, the equipment for communicating with a satellite is expensive. For instance, at both ends of the communications channel in a classical satellite system, combined up-link and down-link antennas are required to enable two-way communications. An uplink antenna and allied electronics is costly and may approach $300,000 per installation. The downlink receiver is an order of magnitude less expensive. To attempt to distribute CBE data via satellite using a standard installations would require a costly up-link at every site where there exists a substantial user terminal population. Moreover, and equally a problem, the time delay for unilateral data transmission through a stationary satellite is approximately 0.25 seconds. Since the data would traverse both the forward and reverse channel directions in such a system, the total time delay would be on the order of 0.5 to 0.6 secs--almost double what is perceived to be acceptable to the user.
Accordingly, it is an object of this invention to provide an improved CBE system wherein communications are accomplished in the most economical manner.
It is another object of this invention to provide an improved CBE system which employs satellite data transmission and still retains an optimum speed of response for the user.
It is a further object of this invention to provide a central computer-based CBE system wherein a plurality of remote CBE sites may be served economically via satellite from a central site.